The present invention relates generally to the field of photonics, and more particularly to photonic integrated circuits (PICs).
The field of photonics has long been touted as having the capacity to revolutionize telecommunications and computing, and to a limited extent, it has delivered on its promise. Indeed, discrete optical devices such as lasers and photodetectors have been developed and greatly increased the capacity of telecommunications systems. Nevertheless, many advances expected of photonic technology have yet to materialize. Notably, photonic integrated circuit (PIC) technology has been slow to develop. Generally, a PIC is a combination of photonic devices in a circuit on a single substrate to achieve a desired function. For example, a PIC may comprise lasers, receivers, waveguides, detectors, semiconductor optical amplifiers (SOA), gratings, and other active and passive semiconductor optical devices on a single substrate. While discrete photonic devices are extremely valuable in their own right, many of the great improvements heralded by photonics can become reality only through the combination of photonic devices in PICs. Despite the benefits expected of PICs, a truly versatile and robust technology for implementing them has not previously been developed.
To date, many photonic integration technologies have been pursued to greater or lesser success. Of the demonstrated methods for forming PICs, the most versatile have been selective area regrowth, selective area growth, and quantum well intermixing. Selective area regrowth technologies are the most sophisticated, with considerable effort dedicated to engineering complex, tunable wave division multiplexing (WDM) sources for fiber optic networks. While these tunable lasers exhibit very high performance, including broad wavelength tunability, temperature stability, and pure spectral emission, they nevertheless require several regrowth steps (on the order of approximately 5 to 7), with precise layer etching of vertical tapers and gratings between each growth. Furthermore, the fabrication steps employed after the multiple growths add significant complexity and cost to the finished chip. Hence, while performance characteristics of such a device (excluding cost) are suitable for sophisticated applications such as wave division multiplexing (WDM), it is unlikely that selective area regrowth processes can be mastered by more than one or two highly specialized laboratories. This suggests that the process will not ultimately result in the rapid design and deployment of the broad range of low cost components demanded by the rapidly expanding digital and analog WDM system architectures. In effect, each selective area re-growth structure must be specialized for a particular application. This increases the design cycle and chip costs to the extent that the integration technology becomes the bottleneck in pacing the growth and deployment of WDM systems.
In contrast, quantum well (QW) intermixing technology lends itself to more rapid demonstration of photonic devices. In this technology area, multi quantum wells (MQWs) are locally disordered by diffusion to create a new alloy whose bandgap is blue-shifted with respect to the MQW light-generating region. While conceptually simple, this technique is rather limited in its scope in that it is used primarily for integrating lasers and waveguide-like structures. Its utility for realizing more complex PICs (incorporating detectors, low voltage modulators, or tunable lasers, for example) is considerably less promising, and hence cannot be widely used for many components unique to WDM architectures. Further, quantum well intermixing has been used primarily for AlGaAs/GaAs systems, and is considerably less mature for InGaAsP-based devices widely used for long wavelength optical communications applications.
Selective area growth, where quantum well dimensions are determined by the width of stripes pre-etched in a growth mask (such as silicon nitride), has similar limitations to those experienced by quantum well intermixing. While growth in wide regions results in red shifting of the effective bandgap of the MQW compared to that grown in narrow regions, this process confines the design only to those devices with equal numbers of QWs. However, a modulator section tends to operate best with 5-10 QWs, while lasers require 1-3 QWs to maximize gain while minimizing threshold current. Once again, this represents a simple but limited approach to integration.
Thus, existing methods for designing and fabricating photonic integrated circuits have significant limitations. Accordingly, there is a need for integration technologies that are robust, truly versatile, and easily implemented on a large scale.
Applicants disclose herein improved photonic integrated circuits. More particularly, disclosed herein are PICs integrated according to asymmetric twin guide (ATG) design principles. In U.S. patent application Ser. No. 09,982,001, filed on Oct. 18, 2001, entitled xe2x80x9cTwin Waveguide Based Design for Photonic Integrated Circuits,xe2x80x9d the contents of which are hereby incorporated by reference in their entirety, there is disclosed a modified twin guide (TG) structure, referred to as an asymmetric twin waveguide (ATG), which addresses many of the performance problems commonly associated with conventional TG structures. The ATG design significantly reduces modal interference by substantially confining different modes of light to propagation in different waveguides. This is accomplished in part by designing the waveguides that are comprised in the twin waveguide structure such that the mode of light that primarily propagates in each waveguide has a different effective index of refraction. The asymmetric waveguides may be laterally tapered to reduce coupling losses by resonant or adiabatic coupling of the optical energy between the first and second waveguide. The asymmetric waveguide design significantly reduces interaction between optical modes and thereby allows for specialization of waveguides to form optical devices.
The ATG design is a versatile platform and has resulted in the development of photonic devices suitable for large-scale commercial use. For example, in U.S. patent application Ser. No. 09/717,851, filed on Nov. 21, 2000, entitled xe2x80x9cPhotonic Integrated Detector Having a Plurality of Asymmetric Waveguides,xe2x80x9d the contents of which are hereby incorporated by reference in their entirety, there is disclosed a photo-detector device based on the asymmetric waveguide design. Similarly, in U.S. patent application Ser. No. 09/891,639, filed on Jun. 26, 2001, entitled xe2x80x9cAsymmetric Waveguide Electroabsorption-Modulated Laser,xe2x80x9d the contents of which are hereby incorporated by reference in their entirety, there is disclosed a semiconductor optical amplifier (SOA) and laser based on the asymmetric waveguide design.
Thus, several photonic devices that employ ATG design principles have been developed. Applicants have noted that the attributes that make ATG design principles attractive for use in discrete optical devices, likewise make it suitable for use in combining optical devices in complex photonic integrated circuits. Disclosed herein are several exemplary monolithic photonic circuits integrated using asymmetric waveguides. Specifically, exemplary PICs disclosed herein include wavelength converters, integrated array waveguide grating (AWG) and detector arrays, and array waveguide-based channel selectors. The disclosed embodiments illustrate discrete photonic devices operatively interconnected to perform desired functions and integrated on a single substrate. More particularly, the PICs comprise devices for amplifying and generating light, devices for absorbing and detecting light, and devices for transporting, sorting, and modulating light. Notably, at least some of the photonic devices in the PICs comprise at least one waveguide that communicates light with a further waveguide, wherein the mode of light in the waveguide has a different effective index of refraction from the mode of light propagating in the further waveguide. Thus, photonic devices integrated in the PICs incorporate asymmetric waveguide design principles.
Importantly, PICs implemented using asymmetric twin waveguide technology do not have many of the limitations associated with previously existing integration methods. For example, photonic devices and circuits implemented using asymmetric waveguide design principles do not typically require regrowth during the manufacturing process. This allows for standardization of wafer designs and fabrication processes. Furthermore, photonic devices and circuits implemented using asymmetric design principles may be defined on a wafer or chip simply by etching to the various levels between the passive, active, and detector waveguide layers. As a result, arbitrary photonic circuits can be defined on the same wafer simply by locally varying the mask levels as required. This unique capability is similar to that routinely employed in CMOS circuit fabricationxe2x80x94the wafers employed are of a standard design, as are all of the devices which comprise the circuit, and the circuit architecture is varied at the mask level. Employing the xe2x80x9cCMOS model,xe2x80x9d asymmetric waveguide design affords the system engineer the ability to rapidly prototype and develop new and ever more complex circuit functions. For this reason, ATG is ideally suited to the demands of complex applications such as chip-scale wavelength division multiplexing (CS-WDM) applications.